Add a 'path' config option to junction elements

This makes it possible to depend on a project which is in a subdirectory
of a Git repository.

The error message given when the expected project.conf file is not found
has also been improved. Previously the error would look like this:

    Error loading pipeline: Could not find file at
    /home/sam/.cache/buildstream/build/freedesktop-sdk-junction-rvmn17s2/project.conf

This is giving the path of an internal temporary directory where the foreign
project's source is checked out. The new error gives more information:

    Error loading pipeline: Could not find the project.conf file for junction
    element at freedesktop-sdk-junction.bst [line 1 column 0]. Expecting a
    project at path '.' within git source at freedesktop-sdk-junction.bst [line
    4 column 2]

buildstream/_loader.py

@@ -362,7 +362,18 @@ class Loader():
 
             source._stage(basedir)
 
-        project = Project(basedir, self.context, junction=element)
+        project_dir = os.path.join(basedir, element.path)
+
+        try:
+            project = Project(project_dir, self.context, junction=element)
+        except LoadError as e:
+            if e.reason == LoadErrorReason.MISSING_FILE:
+                raise LoadError(reason=e.reason,
+                                message="Could not find the project.conf file for {}. "
+                                        "Expecting a project at path '{}' within {}".
+                                        format(element, element.path or '.', source)) from e
+            else:
+                raise
 
         loader = Loader(project, [], parent=self, tempdir=basedir)
 

buildstream/plugins/elements/junction.py

@@ -45,6 +45,9 @@ Overview
        machine_arch: "%{machine_arch}"
        debug: True
 
+     # Optionally look in a subpath of the source repository for the project
+     path: projects/hello
+
 .. note::
 
    Junction elements may not specify any dependencies as they are simply
@@ -115,6 +118,7 @@ from buildstream._pipeline import PipelineError
 class JunctionElement(Element):
 
     def configure(self, node):
+        self.path = self.node_get_member(node, str, 'path', default='')
         self.options = self.node_get_member(node, Mapping, 'options', default={})
 
     def preflight(self):